1. Field of the Invention
This invention relates generally to an image forming apparatus configured to form a full-color image by superimposing colors with a one time charge of an image bearing member.
2. Discussion of the Background
An image forming apparatus, such as a printer, a facsimile machine, a copy machine, a plotter, or a printer/facsimile/copy combination machine is known to form an image according to the following electrophotographic process: charging an image bearing member (hereinafter referred to as “photoconductor”), forming an electrostatic latent image, developing the image by adhering powder (e.g. “toner particle”) to the electrostatic latent image, and transferring the toner image to a medium. Further, electrostatic stylus recorders are another type of image forming apparatus which forms a latent image of electric potential difference on an image bearing member. In this type of image forming apparatus, dielectric material is used as the image bearing member.
Currently, for an image forming apparatus which uses an electrophotographic process, there are two different types of processes that superimpose color on color. One type involves rotating one image bearing member four times. During each rotation, the following steps occur: applying a uniform electrostatic charge to the photoconductor, exposing an image, developing the image with any one of the color toners (cyan, magenta, yellow, black), and transferring the developed image to their respective locations on an intermediate transfer member or recording medium. In the second type process, four photoconductors laterally arranged with respect to each other are used to superimpose color on color. For each photoconductor, the following steps occur: applying a uniform electrostatic charge, exposing an image, developing the an image with any one of the color toners (cyan, magenta, yellow, black), and transferring the developed image on each photoconductor to their respective locations on an intermediate transfer member or a recording medium.
However, the one photoconductor/four rotations type process has a slow printing speed and the laterally arranged four photoconductors type process (i.e., the tandem type process) requires a large and complicated structure, and a high cost.
In light of these deficiencies, a third process has been designed. This type of process superimposes color toner on other color toner during a single rotation of one photoconductor (hereinafter referred to as “one photoconductor/one rotation superimposing type process”). There is also a method for superimposing different color toners on the surface of photoconductor by rotating a single photoconductor four times; however, this type of process has a problem of slow printing. In order to distinguish the process of rotating one photoconductor four times and transferring every color toner image per rotation, and the process of superimposing multiple color toners on the photoconductor without transferring every toner image, the former is referred to as one photoconductor/four rotations/transfer type process and the latter is referred to as one conductor/four rotations/superimpose type process.
In the above described one photoconductor/one rotation superimpose type process, by way of example, four sets of devices are arranged on the side of a belt-shaped or drum-shaped photoconductor. Each set forms a toner image on the photoconductor including, cyan, magenta, yellow, and black. Each set has two uniform charging devices (charging apparatuses) that are corona charging devices, an image exposing device (exposing apparatus), and an image developing device (developing apparatus). Unlike in the case of the one photoconductor/four rotations type process or the laterally arranged four photoconductors type process, this process is completed without transferring the image formed on the photoconductor to the recording medium or the intermediate transfer member. That is, the uniform charging, exposing, and developing are performed for the image on the photoconductor, and then the image of the four superimposed colors is formed in the identical position on the photoconductor.
Consequently, four uniform charges and four image exposures are required and hence miniaturization of an apparatus or lower costs is not substantially obtained. Moreover, speed detection and feedback control in order to obtain less color shift is required and hence the cost is elevated.
Consequently, there is another type of exposing method. This method is called “one shot exposing”. In the one shot exposing process, a latent image of three or four colors is written at one time after a single charging step. The one shot process enables formation of an image without color shift, without speed detection, and without feedback controls which require higher cost.
Japanese published examined application 03-43621 (document 1), Japanese Laid open patent 03-202868 (document 2), and Japanese Laid open patent 03-219260 (document 3) disclose multi-layered photoconductors made with materials which have sensitivity to RGB, respectively, for performing the above described one shot exposing.
Further, Japanese Laid open patent 59-121077 (document 4) discloses a three-layered photoconductor including a photo-sensitive layer which is covered by a transparent insulating layer. Japanese published examined application 59-034310 (document 5), and Japanese Laid open patent 60-225855 (document 6) disclose a mosaic photoconductor which has filter layers of RGB.
In contrast thereto, in Japanese Laid open patent 54-82242 (document 7) a method using an ordinary photoconductor is disclosed. In this method, a latent image which has n levels of electric potential difference is formed by exposing an image, and the latent image is developed as superimposing toners using different developing biases with different toner according to the level. In this instance, as shown in FIG. 8, on the first developed toner, three other toners are formed. However, document 7 explains that the first developed toner of each level comes to the top after the transferring process so that a multi-color image made of the toner color is realized.
Moreover, in the field of conventional image forming apparatuses, the following documents are known. Japanese Laid Open Publication No. 2003-202752 (document 8) discloses a developing device that transports toner with a phase-shift electric field for developing, and Japanese Patent No. 3385008 (document 9) discloses an example of a charging device that charges with a scorotron charging device.
As mentioned in documents 1-6, the above-described apparatuses require use of special kinds of photoconductors. Consequently, these apparatuses are extremely expensive and are not durable and hence cannot be put into practical use.
In contrast thereto, when using an ordinary photoconductor as described in document 7, when using dry type toner, upper and lower toner layers mix and are fused for fixing. As a result, in view of the subtractive color process, a color of the upper toner does not reappear. However, developing color using mixed colors is possible, but in that case, all of the toners become mixed colors, except for the fourth and last one. Consequently, the original color of these toners cannot reappear and forming the full color image is impossible.